The present invention is with respect to a radial fan made up of a spiral housing and a radial impeller therein, the impeller bearing blades running between a support plate and a cover plate having a rounded guide overlapping an inlet cowl of the fan. The blades are designed in the form of airfoils (for example like the section of an airplane wing) having inner edges that are at a slope in relation to the axis of turning of the radial impeller. The inner diameter of the blades becomes smaller from the cover plate to the support plate and the ratio of the mean inner diameter of the blades to the outer diameter thereof is about 0.7 to 1.
Such radial fans, that have come to be used in the art, may be looked upon as the last stage in a process of development whose purpose was that of designing such fans to have the highest possible power density or power to size ratio while at the same time having a good efficiency and an overload-proof characteristic curve. In giving a specification of such a fan dimensionless numbers or coefficients have been widely used, that take into account the well-known relation between the volumetric flow V and the increase in pressure .DELTA.p.sub.t and the diameter and the speed n of turning of the impeller and the density .rho. of the medium to be impelled. The volume number defined as ##EQU1## and the pressure number ##EQU2## put an end to these dependencies and made possible a direct comparison between radial fans with different size and performance data. A radial fan may be described by a characteristic curve in the form of .psi..sub.t against .phi.. The optimum conditions of operation are produced at an optimum point characterized by the pair of values .psi..sub.topt, .phi..sub.opt, in which the efficiency .eta. of the radial blower is at its maximum .eta..sub.max. As a measure for the compactness of the radial fan the power density at the optimum point, that is to say the product of .psi..sub.topt and .phi..sub.opt may be used. Measured in terms of these magnitudes surprising results may be produced with the said form of fan in keeping with the prior art. At an efficiency of .eta..sub.max of 0.85 .psi..sub.topt is 0.91 and .phi..sub.opt is equal to 0.2 so that the power density takes on a value of 0.182. This high power density, which comes near to the power density of a drum impeller fan with forwardly curving blades, is even as such representative of a very compact radial fan. For one and the same field of use it makes it possible for a fan to be fitted with an impeller having backwardly curved blades or with a drum impeller, while the size of the spiral housing and the diameter of the impeller are kept unchanged in size. This in turn makes it possible for the fans to be standardized and to be mass produced on a large scale, so that one may say that a great step forward is now possible in the ventilating and air conditioning arts.